Low pass filters are often used to recover the DC component of a signal having both AC and DC components. The present invention relates generally to improvements in the filtering methods and apparatus applicable to such recovery. In one preferred embodiment the invention is of particular utility in applications where the value of the DC voltage to be recovered can change either abruptly or slowly over a period of time, and where the change can be estimated as it occurs. In one particular application of the invention an abruptly changing DC voltage to be produced is available with reduced filter settling time. This result obtains even in cases where the filter is of the multi-pole variety that would normally have settling times of many time constants. With the aid of the invention one actual size pole 200 Hz low pass filter settles to within one part per million (ppm) in only 40 milliseconds. Without the aid of the invention the same filter requires three seconds for the same degree of settling.
Consider the following example of an actual application of the invention. One type of high resolution digital-to-analog converter (DAC) employs a two-pole switching mechanism connected between an accurate reference voltage and ground. The switch is toggled at a regular rate, producing a waveform having both AC and DC components. The ratio of the times of connection between the reference voltage and ground accurately determines the DC component. The output of the switch is applied to a filter to remove the unwanted AC component.
In such a DAC stringent demands are placed upon the filter if the DAC is to be of high accuracy and also have a low settling time. In particular, the phenomenon of dielectric absorbtion in the capacitors of the filter can disturb the filter's output for a relatively long period of time subsequent to a change in output voltage from the DAC. In accordance with the invention described below such disturbance (and its associated increase in settling time) can be avoided by reducing to a minimum the DC voltage across the capacitors in the filter. This may be done by operating the filter between the DAC output and a voltage that approximates the DAC output, instead of between the DAC output and ground. The approximating voltage may be obtained from a less accurate high-speed second DAC.